I. Field of the Invention
The present invention pertains to rechargeable hydrogen storage batteries and methods for making the same. More particularly, the present invention pertains to rechargeable hydrogen storage batteries having a negative electrode with an active hydrogen storage material and a nickel positive electrode.
II. Description of the Background
Rechargeable batteries with high energy density, high capacity and long cycle life are highly desirable. Two types of alkaline rechargeable batteries commonly used are the Ni—Cd (nickel cadmium) type and the Ni—MH (nickel metal hydride) type. In both types of batteries the positive electrodes are made with an active nickel hydroxide material while the negative electrodes are different.
Ni—MH cells operate by a very different mechanism than Ni—Cd cells. Ni—MH cells utilize a negative electrode that is capable of reversible electrochemical storage of hydrogen, hence the term hydrogen storage battery. The negative and positive electrodes are spaced apart in an alkaline electrolyte. Upon application of an electrical potential across a Ni—MH cell, the active material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical discharge of a hydroxyl ion, as shown in equation 1.M+H2O+e−M−H+OH−  (1)The negative electrode half-cell reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and release an electron through the conduction network into the battery terminal.
The reactions that take place at the positive electrode of the Ni—MH cell are shown in equation 2.Ni(OH)2+OH−NiOOH+H2O+e−  (2)
The use of nickel hydroxide as a positive active material for Ni—MH batteries is generally known. See for example, U.S. Pat. No. 5,523,182, issued Jun. 4, 1996 to Ovshinsky et al., entitled “Enhanced Nickel Hydroxide Positive Electrode Materials For Alkaline Rechargeable Electrochemical Cells”, the disclosure of which is herein incorporated by reference. In U.S. Pat. No. 5,523,182, Ovshinsky et al. describes a positive electrode material comprising particles of nickel hydroxide positive electrode material and a precursor coating of a substantially continuous, uniform encapsulant layer precipitated on the particles to increase conductivity and resistance to corrosion products.
Several forms of positive electrodes exist at present and include: sintered, foamed, and pasted type electrodes. Sintered electrodes are produced by depositing the active material in the interstices of a porous metal matrix followed by sintering the metal. Foamed and pasted electrodes are made with nickel hydroxide particles in contact with a conductive network or substrate, most commonly foam nickel or perforated stainless steel coated with nickel. Several variants of these electrodes exist and include plastic-bonded nickel electrodes, which may utilize graphite as a micro-conductor, and pasted nickel fiber electrodes, which utilize nickel hydroxide particles loaded onto a high porosity, conductive nickel fiber or nickel foam. The current trend has been away from using sintered electrodes in favor of pasted electrodes because pasted electrodes can provide significantly higher loading.
Several processes for making positive electrodes are also generally known, see for example U.S. Pat. No. 5,344,728 issued to Ovshinsky et al., the disclosure of which is herein incorporated by reference, where electrodes having a capacity in excess of 560 mAh/cc are reported. The particular process used for making electrodes can have a significant impact on the electrode's performance. For example, conventional sintered electrodes may now be obtained with an energy density of 480-500 mAh/cc. Sintered positive electrodes are constructed by applying a nickel powder slurry to a nickel-plated, steel base followed by sintering at high temperature. This process causes the individual particles of nickel to weld at their points of contact, resulting in a porous material that is approximately 80% open volume and 20% solid metal. The sintered material is then impregnated with active material by soaking it in an acidic solution of nickel nitrate, followed by conversion to nickel hydroxide in a reaction with alkali metal hydroxide. After impregnation, the material is subjected to electrochemical formation. Pasted electrodes may be made by mixing various powders, such as nickel hydroxide particles, material, binders and additives and applying the mixture to a conductive grid.
Production of nickel hydroxide particles is generally known and is typically made using a precipitation reaction, such as the one described in U.S. Pat. No. 5,348,822, issued to Ovshinsky et al., the disclosure of which is herein incorporated by reference. In U.S. Pat. No. 5,348,822, Ovshinsky et al describes producing nickel hydroxide material by combining a nickel salt with a hydroxide to precipitate nickel hydroxide. Like electrode formation, the method for making nickel active material can have a significant impact on properties and performance of the electrode.
Nickel hydroxide material should have high capacity and long cycle life. Excellent results have been found by forming nickel hydroxide with an apparent density of 1.4-1.7 g/cm3, a tap density of about 1.8-2.3 g/cm3, and an average size range of about 5-50 μ. Excellent results have also been found by making the active, nickel hydroxide with a high packing density and a narrow size distribution, such as may be provided with substantially spherical particles having an average particle size of about 10 μm and a tap density of about 2.2 g/cc. Paste made with this kind of active material has good fluidity and uniformity, making it possible to fabricate high capacity, uniformly loaded electrodes. The use of this kind of active material also improves utilization and discharge capacity. However, if process conditions are not carefully controlled, the resulting precipitate may be irregular in shape and have a low tap density. Electrodes formed with low-density nickel hydroxide will lack high capacity and high energy density. Improper process conditions can also produce a powder that is too fine. An very fine powder will increase adsorption of water at the surface of the particles, thereby requiring longer filtering times. Further, if process conditions are not properly controlled, precipitated particles may be formed with an excessively wide particle size distribution (ranging from 1 to hundreds of microns). Nickel hydroxide made with an excessively wide particle size distribution may require additional processing, such as pulverization, to render it useful. For these reasons and others, active powder having a low density, irregular shape and/or poor size distribution is undesirable for use in a high capacity nickel metal hydride battery.
To produce high density, substantially spherical nickel hydroxide powder, carefully controlled process conditions are used to seed and gradually grow nickel hydroxide particles. Although process conditions can vary, generally the process involves combining a nickel salt with an ammonium ion to form a nickel-ammonium complex. The nickel-ammonium complex is then broken down, typically with caustic, to gradually precipitate nickel hydroxide. However, this reaction rate is difficult to control, so methods have been introduced to separate certain steps in the production process. For example, U.S. Pat. No. 5,498,403, entitled “Method for Preparing High Density Nickel Hydroxide Used for Alkali Rechargeable Batteries”, issued to Shin on Mar. 12, 1996, the disclosure of which is herein incorporated by reference, discloses a method for preparing nickel hydroxide from a nickel sulfate solution using a separate or isolated amine reactor. Nickel sulfate is mixed with ammonium hydroxide in the isolated amine reactor to form a nickel ammonium complex. The nickel ammonium complex is removed from the reactor and sent to a second mixing vessel or reactor where it is combined with a solution of sodium hydroxide to obtain nickel hydroxide. The nickel hydroxide particles may then be formed into a pasted electrode with suitable binders, additives, conductive powders, etc. The electrode is then combined with a negative electrode, separator and a suitable electrolyte to form a hydrogen storage battery.
One useful form of hydrogen storage battery is the sealed type. Sealed batteries are particularly advantageous because they are maintenance free. Sealed batteries typically use a small amount of liquid and operate in a starved condition. However, sealed hydrogen storage batteries are vulnerable to degradation during cycling, particularly, during overcharging and overdischarging conditions. For example, overcharging can effect oxidization of the negative active material. Oxidation of the negative active material it turn results in an irreversible loss of negative electrode capacity and an unbalanced state of charge between the positive electrode and the negative electrode. Another problem that can occur is battery venting. Battery venting can occurs primarily from pressure build-up resulting from hydrogen gas generation at a rate that cannot be recombined within the battery at a complimentary rate. When a critical pressure is reached, gas is vented through a battery safety valve. Most of this excess gas evolution occurs during overcharging at the surface of the negative electrode. To further add to the problem, additional hydrogen evolution may occur from localized heating of the negative electrode. Localized heating occurs from the exothermic reaction of oxygen gas recombining with hydrogen stored at the negative electrode. This localized heating in turn lowers the hydrogen evolution potential at the surface of the negative electrode. The end result is build-up and venting of hydrogen gas. This gas venting causes an irreversible loss of cell capacity and an increase in cell impedance.
To reduce the potential for oxidation of the negative active material and minimize gas evolution, current practice is to make hydrogen storage batteries that are positive limited, e.g. have a positive electrode capacity which is smaller than negative electrode capacity. Excess negative capacity prevents the negative electrode from becoming fully charged and ideally permits oxygen produced at the positive electrode to easily recombine at the surface of the negative electrode according to the following reactions:
                              OH          -                →                                            1              4                        ⁢                          O              2                                +                                    1              2                        ⁢                          H              2                        ⁢            O                    +                                    e              -                        ⁢                                                  ⁢                          (                              at                ⁢                                                                  ⁢                the                ⁢                                                                  ⁢                positive                ⁢                                                                  ⁢                electrode                            )                                                          (        5        )                                          MH          +                                    1              4                        ⁢                          O              2                                      →                  M          +                                    1              2                        ⁢                          H              2                        ⁢            O            ⁢                                                  ⁢                          (                              at                ⁢                                                                  ⁢                the                ⁢                                                                  ⁢                negative                ⁢                                                                  ⁢                electrode                            )                                                          (        6        )            
However, positive limiting a battery alone does not prevent premature failure due to complications during overcharge, overdischarge and charging. As a consequence of these complications, oxidation and gas evolution at the negative electrode act to systematically reduce battery cycle life through electrode oxidation, degradation, battery venting, active material disintegration, etc.